1. Field of the Invention
The present invention generally relates to a fuel supply control system for an internal combustion engine, and more specifically, to such a fuel supply control system, wherein a fuel containing alcohol concentration of 0 to 100% is selectively used, that can drive a fuel pump with a controlled voltage value which is variable depending on a monitored alcohol concentration contained in the fuel, so as to allow the fuel pump to supply a controlled amount of the fuel which is determined based on the monitored alcohol concentration contained in the fuel.
2. Description of the Background Art
An alcohol internal combustion engine has been proposed, wherein a gasoline/alcohol, such as methanol, mixture fuel is used in place of a pure gasoline fuel.
In the alcohol internal combustion engine, an alcohol sensor is provided for monitoring alcohol concentration contained in the mixture fuel to control a fuel injection amount and an ignition timing based on a monitored alcohol concentration. Naturally, the fuel injection amount and the ignition timing are both variable depending on the alcohol concentration because an octane value contained in the gasoline fuel and the alcohol fuel is different from each other. Further, an oxygen sensor is provided as in the pure gasoline engine for monitoring oxygen concentration contained in an exhaust gas so as to perform a FEEDBACK or CLOSED LOOP control of the fuel injection amount to maintain an air/fuel ratio of an air/fuel mixture at a stoichiometric value in a predetermined engine driving range.
In case of the pure gasoline fuel, a fuel injection amount T.sub.i is derived based on the following equation (1) which is well known in the art: EQU T.sub.i =T.sub.p .times.L.sub.ambda .times.L.sub.ambda '.times.C.sub.oef +T.sub.s ( 1)
where, T.sub.p is a basic fuel injection amount derived based on an engine speed and an engine load (an intake air flow rate, for example), L.sub.ambda is a FEEDBACK air/fuel ratio dependent correction coefficient derived based on an output signal from the oxygen sensor, L.sub.ambda ' is a learnt correction coefficient derived based on the FEEDBACK air/fuel ratio dependent correction coefficient L.sub.ambda and the corresponding engine speed and engine load, C.sub.oef is a correction coefficient derived based on various engine operation parameters, such as an engine coolant temperature, and T.sub.s is a correction amount derived based on a battery voltage.
By utilizing the above-noted equation (1), the air/fuel ratio of the air/fuel mixture is controlled to be at the stoichiometric value of 14.7 under the predetermined engine driving range in case of the pure gasoline fuel, i.e. in case of the fuel containing an alcohol concentration of 0%.
On the other hand, in case of the pure alcohol fuel, such as the fuel containing a methanol concentration of 100%, the stoichiometric air/fuel ratio is 6.5 as illustrated in FIG. 6, which is less than a half of that of the pure gasoline fuel.
Accordingly, a fuel injection amount T.sub.i of the pure alcohol fuel or the gasoline/alcohol mixture fuel is derived by the following equation (2) which is also known in the art: EQU T.sub.i .times.C.sub.k .times.T.sub.p .times.L.sub.ambda .times.L.sub.ambda '.times.C.sub.oef +T.sub.s ( 2)
where, C.sub.k is an alcohol concentration dependent constant.
As noted above, since the stoichiometric air/fuel ratio of the pure methanol fuel is less than a half of that of the pure gasoline engine, a fuel pump should supply an amount of the pure alcohol fuel to a fuel injection valve twice as much as that of the pure gasoline fuel. In order to satisfy this requirement, a voltage value of 12 V is set to be applied to the fuel pump, while a voltage value of 8 V is sufficient or optimum in case of the pure gasoline fuel. Accordingly, when the voltage value of 12 V is always set to be applied to the fuel pump, it is wasteful in view of electric power supplied when the fuel containing a less alcohol concentration is used for driving the engine, and further, a noise generated due to a constant high speed operation of the fuel pump becomes a problem.